Known techniques pertinent to a heat-generating sheet making use of heat generation accompanying oxidation of oxidizable metal powder with air oxygen include the heat-generating element of sheet form described in Japanese Patent 2572612. The heat-generating element of sheet form is made by a papermaking technique using a composition comprising iron powder, activated carbon, an electrolyte, water, and a fibrous material. The heat-generating element of sheet form is produced by suspending a fibrous material in water, adding iron powder, activated carbon, an electrolyte etc. to the suspension to prepare a stock slurry, converting the stock slurry into a wet sheet by papermaking processing, dewatering the sheet by suction, followed by press forming to reduce the water content to 5% to 65% by weight.
According to the technique disclosed, the solid matter of the slurry such as iron powder and activated carbon, particularly iron powder with a high specific gravity and activated carbon with a large specific surface area have poor fixability on the fibrous material when converted into a sheet form. Such solid matter is apt to be sucked together with water without being held to the fibrous material, which has resulted in a failure to obtain a heat-generating element having increased iron powder and activated carbon contents. That is, the retention was poor.
In order to secure a high filler retention, papermaking technology called “dual polymer system” has been practiced, in which two kinds of flocculants are used in combination to form firm flocks of fibers and filler (see Teruo Watanabe, Polyion Complex—Budomari System no Jisso Keiken, Japan Pulp Paper, Vol. 52, No. 12, pp. 1717-1725 (December 1998), Japan Pulp Paper Research Institute). Examples of known dual polymer systems include a combination of cationic starch and colloidal silica, a combination of cationic polyacrylamide and modified bentonite, a combination of polyethylene oxide and a special phenolic resin, and a combination of anionic polyacrylamide and cationic polyacrylamide.
The dual polymer system is a well-known flocculation technique, in which a negatively charged fiber or filler is electrically neutralized with a salt or a low molecular weight cationic polymer to form soft flocks, and a high molecular weight anionic polymer then crosslinks between the fiber and filler to make hard flocks resistant to mechanical outer forces.
When the dual polymer system is followed, however, it is necessary to form strong and large flocks to achieve a high retention. Although an increase of retention is expected, the resulting sheet tends to have a poor formation, leading to fall-off of the filler and non-uniformity in mechanical strength. Further, the wet paper layer tends to be difficult to separate from a papermaking wire. As a result, the stock components are liable to remain on the wire, or the wet paper layer is apt to break when transferred from the wire to a drier, which makes continuous papermaking difficult.
If the size of the flocks is made smaller to prioritize proper papermaking, the formation is improved, but resulting in low retention and poor productivity.